This research was designed to test the adequacy of H. Tajfel's (1978, Differentiation between social groups: Studies in the social psychology of intergroup relations, New York/London: Academic Press) theory of social identity for predicting cooperative behavior among members of ethnically mixed groups. Equally divided by sex, subjects were 24 Anglo-American and 24 Hispanic undergraduate students attending the University of California, Riverside. Subjects were exposed to cooperative and competitive feedback in four-member, same-sex groups which varied in the ratio of Anglo to Hispanic subjects to create two conditions—a 3 to 1 versus a 1 to 3 in-group to out-group composition. Results showed that members of both ethnic groups were equally cooperative when their own group was in the numerical majority (low-salience condition). In the high-salience condition, however, when their own group was in the numerical minority, Hispanic subjects were significantly more competitive than Anglo subjects. In addition, while Hispanics were significantly more competitive in the minority than in the majority condition, Anglos showed a slight tendency to be less competitive in the minority than in the majority condition. These results are discussed in light of social identity theory and its application to the study of interethnic cooperation/competition in heterogeneous groups.